Plan S is an initiative for open access publication coordinated by Coalition S. The central requirement is that research funded by public grants from 2021 and after must be published under open access licenses. Specifically research should preferably be published with a Creative Commons Attribution license (CC BY).
This allows not only the public to access information that has been paid for by the public, but allows researchers to retain copyright over their work. Furthermore publication fees and other financing will for the main part be covered by public funding. In Norway Plan S has been signed and is administered by the Norwegian Research Council.
Plan S is summed up in ten principles found here: https://www.coalition-s.org/addendum-to-the-coalition-s-guidance-on-the-implementation-of-plan-s/principles-and-implementation/
You can read more about Plan S and Coalition S here: https://www.coalition-s.org/
Norwegian implementation plan for Plan S (in Norwegian): https://www.openscience.no/oa-i-norge/plan-s